ACT Practice Math Questions: Decimals

By Lisa Zimmer Hatch, Scott A. Hatch

If working with a lot of zeroes and decimals makes you feel faint, then you’d better bring some smelling salts on test day, because the ACT Math exam contains a lot of questions where you have to work with decimals.

The following practice questions will test your skills (and mettle) at converting a number from scientific notation to decimal form, and finding the difference between two decimal numbers so that you can compare the results.

Practice questions

  1. The value of 3 – 0.5 is how many times the value of 1 – 0.5?

    A. 2
    B. 4.5
    C. 5
    D. 5.5
    E. 6

  2. Which of the following is equal to
    ACT_0301
    A. 2,430
    B. 243
    C. 0.0243
    D. 0.000243
    E. 0.0000243

Answers and explanations

  1. The correct answer is Choice (C).
    To solve this relatively easy basic operation problem, first find the two values you need to compare:
    3 – 0.5 = 2.5
    1 – 0.5 = 0.5
    To find out how many times 0.5 goes into 2.5, just divide:
    You know that 25 divided by 5 is 5, so 0.5 goes into 2.5 five times, which means that 2.5 is 5 times greater than 0.5, and 3 – 0.5 is 5 times greater than 1 – 0.5.
    If you picked Choice (A), you found the difference between the two values instead of the quotient.
  2. The correct answer is Choice (E).
    When converting a number in scientific notation, the easiest trick is to just move the decimal to the right the same number of places as the power of the 10. In this case, moving the decimal to the right –5 positions is the same as moving it to the left 5 positions. Write the number with extra zeros on each end and no decimal point in your test booklet. The number looks something like this:
    000002430000
    Place the decimal in its original position between the 2 and the 4 and move it to the left 5 places to come up with the answer 0.0000243.